Add namespace style config to .editorconfig (#2220)

Author: idg10

.editorconfig

@@ -255,4 +255,7 @@ csharp_space_between_square_brackets = false
 
 # Primary constructors
 # We find these something of a mixed bag.
-dotnet_diagnostic.IDE0290.severity = none
+dotnet_diagnostic.IDE0290.severity = none
+
+# Namespace style.
+csharp_style_namespace_declarations = block_scoped